This artist concept shows the Voyager 1 spacecraft entering the space between stars. Interstellar space is dominated by plasma, ionized gas (illustrated as brownish haze), that was thrown off by giant stars millions of years ago. (NASA/JPL-Caltech)

September 12, 2013 — NASA's Voyager 1 probe, which was launched from Earth in 1977 on a historic "grand tour" of the outer planets, has made history again as the first of humanity's objects to venture into interstellar space.

In fact, the 36-year-old spacecraft, which is now about 12 billion miles (19 billion kilometers) from the sun, made the crossover in August 2012, a study published on Thursday (Sept. 12) in the journal Science reports.

"Now we have new, key data, we believe this is mankind's historic leap into interstellar space," said Ed Stone, the project's scientist at the California Institute of Technology in Pasadena. "The Voyager team needed time to analyze those observations and make sense of them. But we can now answer the question we've all been asking — 'Are we there yet?' Yes, we are."

Analysis of the data indicates Voyager has been traveling for about one year through plasma, or ionized gas, present in the space between stars. The probe is in a transitional region immediately outside the solar bubble, where some effects from the sun are still evident.

The spacecraft first detected the increased pressure of interstellar space on the heliosphere, a bubble of charged particles surrounding the sun that reaches far beyond the outer planets, in 2004. Scientists ramped up their search for evidence of the probe's interstellar arrival, knowing the analysis and interpretation of the data could take months or years.

At 11.6 billion miles (18.7 billion kilometers) away from Earth (as of Sept. 12, 2013), the Voyager 1 spacecraft is the most distant man-made object. It passed the previous record holder, Pioneer 10, in 1998. (NASA/JPL-Caltech)

Voyager 1 does not have a working plasma sensor, so a different way was needed to measure the probe's plasma environment to make a determination of its location.

A coronal mass ejection, or a massive burst of solar wind and magnetic fields, that erupted from the sun in March 2012 provided scientists the data they needed. When the burst eventually arrived at Voyager 1's location 13 months later in April 2013, the plasma around the probe began to vibrate like a violin string.

On April 9, Voyager 1's plasma wave instrument detected the movement. The oscillations' pitch helped determine the plasma's density. The particular oscillations meant the probe was bathed in plasma more than 40 times denser than what they had encountered in the outer layer of the heliosphere. Density of this sort is expected in interstellar space.

The science team reviewed its data and found an earlier, fainter set of oscillations in October and November 2012. Through extrapolation of measured plasma densities from both events, the team determined Voyager 1 first entered interstellar space in August 2012.

"We literally jumped out of our seats when we saw these oscillations in our data — they showed us the spacecraft was in an entirely new region, comparable to what was expected in interstellar space, and totally different than in the solar bubble," study lead author Don Gurnett of the University of Iowa, principal investigator of the Voyager 1 plasma wave instrument, said in a statement. "Clearly we had passed through the heliopause, which is the long-hypothesized boundary between the solar plasma and the interstellar plasma."

The new plasma data suggested a timeframe consistent with abrupt, durable changes in the density of energetic particles that were first detected on Aug. 25, 2012. The Voyager team generally accepts this date as the date of interstellar arrival. The particle and plasma changes were what would have been expected during a crossing of the heliopause.

"The team's hard work to build durable spacecraft and carefully manage the Voyager's limited resources paid off in another first for NASA and humanity," Suzanne Dodd, project manager at the Jet Propulsion Laboratory (JPL) in Pasadena, Calif., said. "We expect the fields and particles science instruments on Voyager will continue to send back data through at least 2020. We can't wait to see what the Voyager instruments show us next about deep space."

Voyager 1 and its twin, Voyager 2, were launched 16 days apart in 1977. Both spacecraft flew by Jupiter and Saturn. Voyager 2 also flew by Uranus and Neptune. Voyager 2, which was launched before Voyager 1, holds the record as the longest continuously operated spacecraft and is today about 9.5 billion miles (15 billion kilometers) away from the sun.

The general locations of Voyager 1 and Voyager 2 are shown in this illustration at the edge of the heliosphere, the bubble created by solar wind. (NASA/JPL-Caltech)

Mission controllers send to or receive data from Voyager 1 and Voyager 2 every day, though the emitted signals are very dim, at about 23 watts, or the power of a refrigerator light bulb. By the time the signals get to Earth, they are a fraction of a billion-billionth of a watt.

Data from Voyager 1's instruments are broadcast to Earth typically at 160 bits per second, and captured by 34- and 70-meter NASA Deep Space Network stations. Traveling at the speed of light, a signal from Voyager 1 takes about 17 hours to travel to Earth.

"[Voyager 1] has boldly gone where no probe has gone before, marking one of the most significant technological achievements in the annals of the history of science, and adding a new chapter in human scientific dreams and endeavors," said John Grunsfeld, associate administrator for science at NASA. "Perhaps some future deep space explorers will catch up with Voyager, our first interstellar envoy, and reflect on how this intrepid spacecraft helped enable their journey."

It not known when the Voyager 1 spacecraft will reach the undisturbed part of interstellar space where there is no influence from the sun. It is also not certain when Voyager 2 will cross into interstellar space, but scientists believe it is not very far behind.